Production of cellulose



Patented Aug. 10, 1954 UNITED STATES ATENT OFFICE PRODUCTION OFCELLULOSE Britain Spon- British Great No Drawing. Application March 1,1950, Serial No. 147,157

Claims priority, application Great Britain March 2, 1949 Claims.

This invention relates to the production of cellulose and its conversioninto cellulose esters and others of good quality.

The main sources of cellulose for conversion into cellulose esters andothers are cotton linters and pulps obtained from wood. Of these, cottonlinters are normally the purer form of cellulose. However, even theyrequire a certain degree of purification, the ordinary practice being toboil the linters under pressure with a dilute alkali solution, e. g. acaustic soda solution of concentration below 5% and usually about 3%,and then to bleach them. While cellulose acetate, for example, of goodquality can by careful working be obtained from cotton which has beengiven this treatment, it is generally not ideal from the moulders pointof view, in that durin moulding a certain amount of colour is nearlyalways developed, presumably as a result of the high temperatures towhich the cellulose acetate is raised during the moulding operation. Itis an object of the present invention to produce a cellulose which iscapable of being converted into cellulose acetate and other cellulosederivatives which can be moulded with as little colour change aspossible.

According to the invention, cotton is given a purification treatmentwhich comprises the step of subjecting it to the action of an aqueouscaus tic alkali solution of concentration 15 %-25%, preferably 15%-20%,at a temperature at Which it becomes partially, but not completely,mercerized. We have found that as a result of such treatment, the cottoncan be acetylated to give a product which on moulding developsconsiderably less colour than when the cotton has been treated in theconventional way. The treatment with the 15%-25% caustic alkali solutionpreferably precedes or follows a pressure boil with caustic alkali ofconcentration below about 5%, especially a boil with a 2%5% causticalkali solution at a temperature of 12 l180 C. It is advantageous alsoto bleach the cotton either before or after the treatment with the 15%-25% caustic alkali solution, for example with chlorine or with sodiumor calcium hypochlorite in acid solution or, especially if aparticularly high viscosity product is required, with sodium chlorite.Both the pressure boil and the bleach may be carried out in one or moreoperations.

It is, of course, well known that complete mercerisation of cellulose bymeans of an aqueous caustic alkali solution takes place only below adefinite temperature, which depends on the concentration of the alkalisolution. Above this temperature is a range within which partialmercerisation occurs. A graph relating temperature and caustic sodaconcentration to the degree of mer- Whether it is partial or complete)is given in Cellulose and Cellulose Derivatives by Emil Ott (New York1943) on page 274. For a caustic soda solution the temperature belowwhich mercerisation is complete is about 27 (3.; for caustic Soda it isabout 40 (3.; and for caustic soda it is about 48 C. In the practice ofthe invention it is advantageous to treat the cotton at a temperaturewhich is higher than the maximum at which complete mercerisation occursby 5 C. or more; thus a 15% caustic soda solution is preferably used ata temperature of about 32 C. or higher, and a 20% solution at about 45C. or higher. The temperature should not be so high that little or nomercerisation occurs, and it is preferred not to exceed a temperaturewhich is about 25-30 C. above the maximum at which completemercerisation occurs; thus it is preferred to work at a temperaturewithin a range varying from about 32-50 C., preferably 35-45 C., with a15% caustic soda solution, to about -65 C., preferably eff- C. with a20% solution, and about 5375 C., preferably 5565 0., with a 25%solution. Very good results are obtained by using a 16%18% caustic sodasolution at about 37l5 C.

The treatment may be carried out by immersing the cotton in the causticalkali solution at the desired temperature, if desired with stirring,for a period for instance between about 15 minutes and 5 hours andespecially for about 20-90 minutes. Other methods may however beemployed; for example the alkali solution may be sprayed on the cotton,though it may then be less easy to obtain a thorough and uniformtreatment. The alkali solution may contain a small proportion Turkey redoil (as may also a dilute alkali solution used in a pressure boil asdescribed above). If desired, oxygen may be excluded during thetreatment so as to reduce to the minimum any At the end of the desiredtreatment period, the alkali may be run ofi or otherwise removed fromthe cotton, which may then be washed substantially free from alkali anddried. For example it may be washed with water until the washings nolonger react alkaline, or it may be given one or more washes with adilute acetic or other acid solution, preferably preceded and followedby washing with water. For the various washes mains water may be used,but it is generally advantageous to employ distilled water or waterwhich has been demineralised by an ionexchange process, especially inthe later stages of the washing. Finally, if desired, the cellulose maybe washed with an organic liquid, especially acetone, to remove water,or it may be completely or partly freed from water by some other method,e. g. by means of a current of warm air, preferably without raising itstemperature above about 60 C.

Although it is usually suflicient to subject the cotton to a singletreatment with the l%-25% caustic alkali solution, it may, if in anyparticular case it appears desirable, be given two or even more suchtreatments, preferably with a total duration of 20 minutes or more,either under the same or under somewhat different conditions fallingwithin the specified limits. Such treatments may be consecutive, or theymay be separated in the order of operations by other treatments, e. g.the dilute alkali pressure boil or bleaching operation already referredto. in any case it is advisable to wash the cellulose thoroughly aftereach treatment, though it is not essential to remove all traces ofalkali or of course to dry the cotton before it is subjected to afurther stage of the purification.

Cotton treated in accordance with the inven tion may be used in theproduction of organic derivatives of cellulose, and especially formaking cellulose esters and ethers which are to be moulded. For exampleit may be acetylated by known methods, preferably after a pretreatmentwith glacial acetic acid with or without a small proportion of anacetylation catalyst, e. g. sulphuric acid or perchloric acid. Whenacetylation is substantially complete, the product may be ripened toreduce the acetyl content and modify the solubility properties of thecellulose acetate as may be desired; advantageously the ripening may beperformed at a high or fairly high ten.- perature after neutralisingpart or all of the acetylation catalyst. If desired the acetylationsolution may be filtered before or, preferably, after the ripening. Theripened cellulose acetate may be precipitated in known manner, as bystirring in dilute acetic acid, and then thoroughly washed. It willgenerally be advisable to give the precipitated and washed celluloseacetate a separate stabilising treatment, for example by warming it withvery dilute sulphuric acid or boiling it under pressure withdemineralised or distilled water, and also if desired with an aqueousalcohol.

Cotton treated by the new process is also very suitable for conversioninto other cellulose esters, e. g. cellulose propionate, cellulosecrotonate, cellulose butyrate, cellulose stearate and cellulosebenzoate, and also into mixed esters as for example cellulose acetatepropionate, cellulose acetate butyrate and cellulose acetate stearate.The cellulose esters may, if desired, be purified by fractionation orextraction methods, in order still further to reduce the tendency tocolour formation. When the cotton is to be used in the production ofcellulose ethers, e. g. ethyl cellulose, it is not as a rule necessaryto wash out all traces of alkali, but it is very desirable to washthoroughly enough to ensure that substantially all the impuritiesdissolved by the alkali are removed.

The invention is illustrated by the following example; the parts givenare in all cases parts by weight.

Example Cotton linters were given a boil at about 50 p. s. i. g. with a3% caustic soda solution, washed thoroughly, bleached with calciumhypochlorite in acid solution, and again thoroughly washed. The bleachedlinters were immersed for 30 minutes in a 17% caustic soda solution at atemperature of C.; the caustic soda solution was then run off and thelinters washed several times with mains water and then once with a 0.1%acetic acid solution, again with mains water and then three times withdemineralised water. Finally the linters were washed once with acetoneand allowed to dry at room temperature.

100 parts of the purified linters were activated by treatment with partsof acetic acid at room temperature for 2 hours, and then cooled to 5 C.and introduced into about 600 parts of an acetylation mixture, cooled to0 0., consisting of 40% of acetic anhydride and of acetic acid andcontaining 12 parts of sulphuric acid. When acetyla-tion was completethere was added a magnesium acetate solution or concentration and inamount such that all unchanged acetic anhydride was hydrolysed to aceticacid, 100 parts of additional water was added to the acetylationsolution. and 11.5 parts of the sulphuric acid was neutralised. Thesolution was then ripened at a temperature of -95 C. for about 2 /2hours, after which the cellulose acetate was precipitated by stirring indilute acetic acid. The precipitate was washed with mains water, andthen stabilised by heating with distilled water for 1 hour to a pressureof 40 p. s. i. g. It was then washed to neutrality with distilled waterand dried. The product had an acetyl value of 56.5% on being moulded itdeveloped considerably less colour than did cellulose acetate made inexactly the same way except that the treatment with the 17% caustic sodasolution was omitted.

The process described above can be modified by subjecting the lintersfirst to the treatment with the 17% caustic soda solution, then to thepressure boil and then to the hypochlorite bleach, care being taken towash them very thoroughly affter the bleach, as well as after the otherpurification stages.

If desired about 1% (on the solution) of Turkey red oil can be employedin either or both of the alkali treatments.

While the invention has been particularly described by reference toprocesses in which the cotton is not only treated with the 15%-25%caustic alkali solution, but is also given a pressure boil with a dilutecaustic alkali solution and a bleach, the treatment with the l5%-25caustic alkali solution may be employed alone, or it may be supplementedby other purification steps. However, the processes described aboverepresent the preferred method of putting the invention into practice.

In the claims, in order to distinguish clearly between the l5%-25%alkali solution and the dilute alkali solution used in a preceding orfollowing pressure boil, the former will be referred to as a strongsolution and the latter as a weak solution. It will be understood thatthe adjectives strong and weak do not import any limitations into theclaims that are not already expressed in numerical terms.

Having described our invention, what we desire to secure by LettersPatent is:

1. In the purification of cotton linters by a process which includesboiling the linters under pressure in a 2-5% caustic alkali solution andbleaching the linters, the improvement which consists in giving thelinters a subsequent additional treatment with a caustic soda solutionof concentration -20% at a temperature varying from 3250 C. with a 15%solution to 45-65 C. with a solution.

2. In the purification of cotton linters by a process which includesboiling the linters under pressure in a 2-5% caustic alkali solution andbleaching the linters, the improvement which consists in giving thelinters a subsequent additional treatment with a caustic soda solutionof concentration 15-20% at a temperature varying from 3545 C. with a 15%solution to -55 C. with a 20% solution.

3. In the purification of cotton linters by a process which includesboiling the linters under pressure in a 25% caustic alkali solution andbleaching the linters, the improvement which consists in giving thelinters a subsequent additional treatment with a caustic soda solutionof concentration 16-18% at a temperature of 37-45 C.

4. Process according to claim 1, wherein at least one of the alkalisolutions contains a wetting agent.

5. Process according to claim 1, wherein at the end of the purificationtreatment the linters are washed with aqueous washing liquors, at leastthe last stages of the washing being carried out with watersubstantially free from metal ions.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,087,263 Olsen July 20, 1937 2,317,767 Hofmann Apr. 27, 19432,380,706 Richter July 31, 1945 2,408,849 Haney et a1. Oct. 8, 19462,546,749 Holloway Mar. 27, 1951 FOREIGN PATENTS Number Country Date405,884 Great Britain Feb. 15, 1934 263,938 Great Britain Jan. 6, 1927269,012 Great Britain Apr. 14, 1927 OTHER REFERENCES Ott, Cellulose andCellulose Derivatives, 1943, pages 272-276.

Marsh-Mercerising, Van Nostrand, New York, 1942, page 437.

1. IN THE PURIFICATION OF COTTON LINTERS BY A PROCESS WHICH INCLUDESBOILING THE LINTERS UNDER PRESSURE IN A 2-5% CAUSTIC ALKALI SOLUTION ANDBLEACHING THE LINTERS, THE IMPROVEMENT WHICH CONSISTS IN GIVING THELINTERS A SUBSEQUENT ADDITIONAL TREATMENT WITH A CAUSTIC SODA SOLUTIONOF CONCENTRATION 15-20% AT A TEMPERATURE VARYING FROM 32-50* C. WITH A15% SOLUTION TO 45-65* C. WITH A 20% SOLUTION.